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2017SummerShotcreteEMag

Bridge and Highway Repairs Using Shotcrete and Cathodic Protection By Dennis Bittner and Erik Bertrand It was once believed that microsilica-enhanced shotcretes could not be used with cathodic protection. This belief centered on the argument that shotcrete was too dense and lacked the necessary permeability to be compatible with cathodic protection. The concern was that the density of the shotcrete inhibited proper flow of current, thus preventing the anodes from properly functioning. However, new techniques are being developed to allow the benefits of cathodic protection and shotcrete to work together. To understand the issues with using low-permeability materials and cathodic protection together, it is helpful to understand how permeability is tested and what those results mean. Permeability is often tested using ASTM C1202, “Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration.” This test method determines the electrical conductance of concrete, thus giving an indication of the concrete’s resistance to penetration by chloride ions. Basically, electrical current is monitored as it passes through 2 in. (50 mm) slices of 4 in. (100 mm) concrete cores. The greater the density of the material being tested, the greater the material inhibits electrical current flow. This method of testing measures conductivity of the sample material in coulombs and provides a correlation with permeability. A highly conductive sample material is determined to be highly permeable and low conductivity implies a lower permeability. Microsilica-enhanced shotcretes have very low conductivity and thus very low permeability, with conductivity measures often below 500 coulombs. Historically, a cathodic protection system using anodes would require the surrounding cementitious material to have a significantly higher conductivity in the 1500 coulomb range. Several techniques have been developed to overcome these conductivity issues, thus allowing the two systems to work together. First, it was determined that encapsulating the anode in a less dense, more conductive material, such as non-shrink grout prior to application of the shotcrete would minimize the adverse effects the shotcrete has on the anode. Non-shrink grout is used to “pancake” the anode, surrounding it with a material that does not interfere with the anode’s ability to work correctly. Another technique developed was to redesign the anode shape so that it could Cathodic protection is now offered in various shapes and sizes, some of which are more conducive to shotcrete application 20 Shotcrete | Summer 2017 www.shotcrete.org


2017SummerShotcreteEMag
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